Projection display systems can include a source of light, illumination optics, an image-forming device, projection optics and a projection screen. The illumination optics collect light from a light source and direct it to one or more image-forming devices. The image-forming device(s), controlled by an electronically conditioned and processed digital video signal, produces an image corresponding to the video signal. Projection optics then magnify the image and project it onto the projection screen. White light sources, such as arc lamps, in conjunction with color wheels have been and still are used as light sources for projection display systems. However, recently light emitting diodes (LEDs) have been introduced as an alternative source of light. Some advantages of LEDs include longer lifetime, higher efficiency and superior thermal characteristics.
Examples of image-forming devices frequently used in digital light processing systems are digital micro-mirror devices (DMDs) and liquid crystal type imaging devices such as liquid crystal on silicon (LCoS) and high temperature polysilicon (HTPS) imagers. These imagers feature an array of individually controllable elements, with the states of the elements being individually controlled by the data loaded in a memory cell associated with each element, to steer reflected or transmitted light and spatially map a pixel of video data to a pixel on a projection screen. Light reflected or transmitted by an element in an ON state passes through the projection optics and is projected onto the screen to create a bright field. On the other hand, light reflected or transmitted by an element in an OFF state is not passed through the projection optics, which results in a dark field. A color image may also be produced using these imagers, such as by utilizing color sequencing or alternatively by using three imagers, one for each primary color (red, blue and green).